The Solar Optical Telescope

Home , Solar telescope, Solar transition region

NASATM109240

(NAS.A- TM- 109240) TELESCOPE (NASA)

NASA The Solar Optical Telescope is shown in the Sun-pointed configuration, mounted on an Instrument Pointing System which is attached to a Spacelab Pallet riding in the Shuttle Orbiter's Cargo Bay. The Solar Optical Telescope

- will study the physics of the Sun on the scale at which many of the important physical processes occur - will attain a resolution of 73km on the Sun or 0.1 arc seconds of angular resolution 1

SOT-I GENERAL CONFIGURATION

ARTICULATED VIEWPOINT DOOR PRIMARY MIRROR op IPS SENSORS WAVEFRONT SENSOR - / i— VENT LIGHT TUNNEL-7' ,—FINAL I 7 I FOCUS

IPS INTERFACE

E-BOX SHELF GREGORIAN POD---' U (1 of 4) HEAT REJECTION MIRROR-S

1285'i Why is the Solar Optical Telescope Needed? There may be no single object in nature that mankind For exrmple, the hedii nq and expns on of the solar wind U 2 is more dependent upon than the Sun, unless it is the bathes the Earth in solar plasma is ultimately attributable to small- Earth itself. Without the Sun's radiant energy, there scale processes that occur close to the solar surface. Only by would be no life on Earth as we know it. Even our prim- observing the underlying processes on the small scale afforded ary source of energy today, fossil fuels, is available be- by the Solar Optical Telescope can we hope to gain a profound cause of solar energy millions of years ago; and when understanding of how the Sun transfers its radiant and particle mankind succeeds in taming the nuclear reaction that energy through the different atmospheric regions and ultimately converts hydrogen into helium for our future energy to our Earth. needs, it will be the reaction first discovered, and still Perhaps the best analogy for the radical new perspec- being studied, in the Sun's core that will provide that tive that the Solar Optical Telescope will give us on energy. Meanwhile, the Sun's radiation will continue to solar physics is that of the role of the microscope in drive the circulation of our terrestrial atmosphere and clarifying the nature of blood. Before the invention of will contribute to changes in our climate through subtle the microscope, there was a great deal of speculation on variations in the quantity and the spectrum of that radi- the nature of this vital fluid. Since the discovery of ation. Thus, the Sun remains, in some sense, as impor- blood circulation by the physician Harvey, we have un- tant today as it was to the ancients who worshiped it as derstood that blood somehow is involved both in "feed- a god and who sensed, even if they did not fully under- ing" the body and in removing some of its locally gener- stand, how fundamental the Sun's role is in determining ated waste products. But how? Only with observations the character and viability of our home, the Earth. of blood cells and the determination of their structure Beyond its importance to life on Earth, the Sun is also of and function could our current understanding develop. fundamental importance to stellar astronomy. The Sun is a very For this, the microscope was needed. In a like manner, typical star, and it is the only star that we can study with high the Solar Optical Telescope will provide views of the angular resolution, thanks to its proximity to us This means that solar surface that, for the first time, will permit its bas- we can study physical processes that are of fundamental sig- ic structure, "cellular" or otherwise, to be resolved. nificance in understanding stars in general, on the scale on which The Sun has long been the focus of human attention these processes are actually occurring! The history of astronomy and admiration. The arts attest nobly to this interest. In demonstrates convincingly that much of our understanding of the closing phases of Mozart's famous opera "The Mag- the physics of stars comes from understanding these physical ic Flute." the Sun-priest utters the lines (loosely trans- processes on the Sun first. It is for this reason that the Sun is lated): often called the Rosetta stone of astronomy. 'The Sun's golden rays pierce through the night, The Solar Optical Telescope will be the world's first facility that And scatter the powers of Darkness to flight.' is capable of observing the Sun on the angular resolution of a It is appropriate that we should approach the objects of typical photospheric (lower atmospheric) mean-free path for ra- nature both from the perspective of the arts as well as diation and also on the scale known to characterize major changes from that of the sciences. The Solar Optical Telescope in the gas-dynamic behavior of the atmospheric medium In gives us the opportunity of taking the next great step in simple language, these are the scales on which energy is trans- scientifically understanding the Sun, as well as those ferred in the solar atmosphere; hence, they are the scales on other fascinating astronomical objects of which it is the which the major physical processes that ultimately determine closest member, the stars. the dramatic large-scale behavior of the atmosphere take place. 6.1

I I i 4¼ 1 I 1' 1t¼ I TRANSITION 111111111 i'ii REGION OVER SUPER GRANULE

- - - Mt 5.0 \lP \ c LIMB

SUPER GRANULE FLOW

Schematic cross section of "quiet" solar atmosphere, with supergranulatiofl flow below limb and inhomogeneous magnetic field above limb given as dashed lines. Numbers are Log 10 of the temperature.

Current Picture of the Sun's Atmosphere and Convection Zone The picture of the solar atmosphere and upper con- tions to the overlying atmosphere Since the convection zone vection zone that has developed over the past three dec- is not directly observable in any wavelength, except at its up- ades emphasizes the importance of magnetic fields and per boundary with the photosphere, its deeper structure can inhomogeneities. The solar atmosphere is never entirely be known only from careful studies of oscillations observed at homogeneous at any height. Material upwelling from the the solar surface (solar seismology) and from theoretical mod- deep unobservable regions of the convection zone spills eling. The only hope of understanding solar convection in de- over into the stable photosphere, where two distinct scales of tail, then, depends on using the boundary conditions obtained convective eddies are observed in the granulation and from observations at the surface and solving the differential supergranulation flow patterns. Above the photosphere. Jets equations of gas dynamics. To date, solar observations have of material called spicules shoot up from the top of the yielded a picture of convection in which elongated, hexagon- chromosphere to several thousands of kilometers into the al cells extend to some currently undertermined depth of the corona. Strong, highly localized and essentially vertical convection zone and reveal themselves in the observable magnetic fields extend from the subsurface regions through photosphere as the supergranulation flow pattern. Superim- the photosphere. These strong fields spread out and weaken posed on this larger scale flow is the smaller scale motion ob- in the higher layers, forming large-scale regions of closed fields served at the level of the photosphere and called the granula- similar to those of a dipole, as well as large regions of open tion. Theoretical estimates of flow conditions in the low pho- field lines extending into interplanetary space. Superimposed tosphere predict that this small-scale flow should be turbulent, on this complex pattern of systematic and impulsive flows and and that is largely what is actually observed there. Two other magnetic fields is a spectrum of oscillations and waves on scales of convection have been predicted theoretically, but varying scales, ranging from coherent pulsations of the entire neither has been reliably observed. These are (I) a very large- Sun with periods of up to almost 3 hours, to hydromagnetic scale. "giant cell" with a scale length extending a significant waves localized to the photosphere and chromosphere with fraction of a solar radius, and (2) a "mesoscale" convection, periods on the order of I minute. intermediate between the scales for granulation and super- Underlying this complex atmosphere is the solar convection granulation. zone, in which energy is transported by means of convective The question of the velocity structure of the convec- motions from the core where it is generated by nuclear reaction zone, which is intrinsically interesting, also under- lies one of the most fundamental problems in solar/stel- Let us move up higher into the solar atmosphere 4 lar astrophysics: What is the character of the dynamo again. We now know that it is filled with numerous that generates the Sun's (or a star's) magnetic field? other features whose structure and dynamics are domi- Since the motions of the subphotospheric solar plasma, nated largely by the magnetic field. This occurs be- coupled with the solar differential rotation, are ultimate- cause, above the chromosphere, the energy in the mag- ly responsible for the solar magnetic field, certain gen- netic field exceeds that stored thermodynamically in the eral constraints on the type of dynamo have already gas, reversing the picture that applied in the lower lying emerged from observations of the Sun's surface magnet- regions. Large-scale features, called prominences and ic field. For example, the dynamo must be able to ge- filaments, are well observed structures that are still on- erate the cyclic behavior in the field over a 22-year peri- ly partly understood solar phenomena for which the do- od known as the solar cycle. Unfortunately, current lim- minant parameter appears to be the local magnetic itations in the angular resolution of ground-based data field, which determines the observed behavior. From time prevent our constraining the current dynamo models to time, magnetic energy is transformed into heat by a enough to get an accurate picture of how the dynamo still poorly understood process that is observed in catas- works or of why the scale of the surface magnetic flux trophic events called solar flares. The solar atmosphere tubes is so small. What is needed is the interaction bet- is clearly an ideal laboratory for studying many impor- ween magnetic fields and fluid motions—which occurs tant plasma processes, which are also present in other on the small sub-arc-second scale of the flux tubes. types of stars. Some Scientific Problems for the Solar Optical Telescope I. SOLAR MAGNETIC FIELDS 4. ATMOSPHERIC DYNAMICS The basic structure of the solar magnetic field is now The mass balance between different regions of the solar at- 5 thought to be the flux tube. The scale of these flux tubes in the mosphere has long been a major problem in solar physics. The photosphere is smaller than the resolving power of ground- mass loss inferred from observed upward motions of spicules, based telescopes. The reason why much of the magnetic field whose internal motions cannot be resolved, is more than an in the photosphere is on such a small scale is not understood, order of magnitude larger than the measured mass loss in the although theories that address this problem have been pro- solar wind at the Earth's orbit. The Solar Optical Telescope will posed. The Solar Telescope will permit these theories to be resolve the smaller scale velocity fields that contribute to the checked, by obtaining high resolution data on both the mag- overall mass balance netic fields and the associated gas flows in the magnetic field regions. S. SOLAR FLARES The most difficult problem in the physics of solar flares has 2. CONVECTIVE ENERGY TRANSPORT proven to be identifying a "trigger mechanism" whereby the Models for the velocity structure of the solar convection enormous energy stored in the convoluted ambient magnetic zone are currently being generated. One constraint on these field is suddenly and dramatically released. Since the triggering models is the detailed behavior of the convection at the upper can involve plasma instabilities on unobservably small scales, boundary of this region in the low photosphere. The Solar Op- even for the Solar Optical Telescope, the most productive ap- tical Telescope will help solar physicists to resolve the complex proach has been to constrain the range of possible trigger velocity structure of this observable region. Superior models of mechanisms by observing the plasma changes on the smallest convection zone dynamics will yield superior models for the scale that can be observed. The Solar Optical Telescope will dynamo that generates the Sun's magnetic field. decrease the scale of observable changes by about one order of magnitude. 3. SOLAR ATMOSPHERIC HEATING Because the last of the Orbiting Solar Observatories (OSO-8) 6. INSIGHTS INTO OTHER STARS demonstrated that there was insufficient energy in acoustic The Sun, by its proximity, is much more amenable to detailed waves to heat the solar atmosphere above the chromosphere, study of many fundamental stellar processes than any other star. solar physicists have sought evidence for hydromagnetic and Again and again, processes first observed and understood on plasma processes to heat the higher lying regions of the chro- the Sun are used to explain similar phenomena observed on mosphere, the transition region, and the corona. Many of other stars. The latest example is the extrapolation, over much these processes occur on a scale too small for detailed study of the Hertzsprung-Russell diagram, of what is known about tIle with current telescopes. The Solar Optical Telescope will cor- solar chromosphere, corona and wind. Data from the Interna- rect this situation. tional Ultraviolet Explorer and Einstein Observatories and advances in solar physics from the Solar Optical Telescope can be expected to play a similar role in stimulating stellar astrophysics in the future. The two magnetograms shown above demonstrate the ten- 6 dency for solar magnetic fields (yellowish emission) to be pushed toward continuum lanes (violet) by motion of granules. Solar Magnetic Fields

We have already noted that the perplexing problem of the according to one current picture, flux tubes "collapse" to an stability of small-scale flux tubes in the photosphere requires equilibrium configuration because of "downdrafts," then up- observations of the flux tubes on sub-arc-second scales. Since flows within a flux tube may cause expansion and may be as- these flux tubes are thought to be just below the scale of cur- sociated with magnetic breakdown and diffusion at the edge rent observations, and since definitive observations will have of solar active regions. Also, by associating solar magnetic field to include their associated gas flows, it is clear why the Solar structure with values for the densities and velocities in the so- Optical Telescope is needed to obtain the data needed for lar photosphere, the stability theory would permit inferences checking the theory. Magnetic fields in sunspots will also be to be made about scales for the magnetic elements in other studied at high resolutions, because the best current data stellar atmospheres for which model atmospheres and mean show that the spot itself is composed of a complex, largely ver- magnetic field strengths are available. Finally, sunspots offer tical magnetic field structure in the umbra and more nearly an excellent laboratory for studying strong magnetic fields on horizontal fields in the surrounding penumbra, where convec- other stars. Late M-type stars, for example, are known to have tive flows also appear to be taking place. Solution of the flux- fields of several thousand gauss, comparable to sunspot fields. tube stability problem has far-ranging possible applications. If, Convective Energy Transport

The granulation at the top of the solar convection zone is zone velocity models and as the source of numerous types of the visible evidence of turbulent convection at the upper waves that transport energy to higher lying regions of the boundary between this zone and the convectively stable up- solar atmosphere. These waves can be acoustic waves in the per photosphere. The granulation is important for a number of absence of magnetic fields or hydromagnetic waves where reasons. First, it is necessary to characterize the granulation local magnetic fields both enhance the power generated and flow field accurately in order to subtract it from the aggregate direct the propagation upward. Of critical importance to com- photospheric velocity field, which contains other systematic puting the power generated in both cases are the high-fre- and oscillatory components one wishes to study. Separation quency components of the granulation power spectrum (i.e.. of these different velocity fields, of different origins, is a chal- the small, short-lived elements). These components cannot be lenging task that requires data of very high resolution in both observed with present telescopes. The Solar Optical Telescope spatial and temporal frequencies. The granulation is also in- will be capable of observing them and obtaining the needed trinsically interesting as a boundary condition on convection power spectra. 7

Sacramento Peak photo of solar granulation.

A4 I

r I

Mj C, Sun's ultraviolet continuum seen by HRTS rocket. All of the 8 bright emission is indicative of atmospheric heating. Solar Atmospheric Heating Since 050-8 observations have cast doubt on the hypothe- served with the HRTS sounding rocket ultraviolet spec- sis of wave heating of the solar atmosphere above the trograph/spectroheliograph. The common characteristic of all chromosphere, the hypothesis of heating by the dissipation of of these heating mechanisms is their dependence on sharp electric currents through local magnetic field annihilation has spatial gradients in either the local magnetic field, the local gained many adherents, particularly for the heating of coronal velocity field, or both. In addition, the temporal history of the active-region loops. However, the wave-heating hypothesis is local temperature, and usually of other parameters, must be not entirely dead. In the presence of magnetic fields, many determined. Obtaining all of these data simultaneously with kinds of wave modes are possible. Some of these can pene- the required resolution over a field of view large enough to es- trate through the chromosphere and transition region into the tablish the context of the physical process present is beyond corona, or they can even be generated by turbulence in the the power of any existing instrumentation. The Solar Optical transition region itself, as some of the Skylab observations Telescope, with the complete Combined Instrument Package have suggested. Moreover, a third possibility for heating the as payload, will have this power. corona is by the dissipation of supersonic jets of the type ob- * Skylab image of coronal loops. Mass exchange with chromosphere is likely.

9 Atmospheric Dynamics The question of mass balance between the chromospheric spicules and the solar wind is only one aspect of the dynamic behavior of the Sun's outer atmosphere that requires further attention. Recent observations have revealed a complex pattern of both vertical and horizontal flows in the chromosphere, in some cases with material moving both up and down Skylab X-ray images of active regions. Detaiied structure in association with known features such as "bushes" of spi- breaks up into loops (above). cules (suggesting that the earlier picture that spicules represent only upward motion may be a highly misleading oversimplifi- cation). Very strong downflows, as well as dramatic, high-velocity jets of upward moving material, are also observed in the transition region line of CIV, formed at temperatures of 10 5 K. In addition, analyses of Skylab observations strongly suggest that a complex cycling of material occurs in association with the heating of coronal active regions. The corona is heated and, at very high temperatures. loses radiative cooling efficiency. Con- duction of energy down to the chromosphere takes place in the nresence of the sharp temperature gradient in the transition re- , ^jon, and chromospheric plasma is then heated and "boiled off" up into the corona, where it radiates away the residual excess nnergy from the original heating impulse. All of these processes, as well as others, must be studied at high spatial, spectral, and temporal resolution to clarify the dynamics of the Sun's atmosphere.

Dense active region loop material suspendec in the corona. Solar Flares The Solar Optical Telescope will decrease the scale on which changes can be observed in solar flares by about one order of magnitude, or more in some wavelengths. This increase in angular resolution, with corresponding changes in a more rapid temporal response to the rapidly changing conditions in a solar flare, will permit a much more accurate assessment to be made of the physical conditions within the flare. The strongest spectral line of four times ionized carbon, CIV, formed in the solar transition region at a temperature of 15 K, has been observed with the ultraviolet instrument on the 5MM spacecraft at an angular resolution of 3 arc-seconds. With the Solar Optical Telescope, a factor of 30 increase in this angular resolution becomes possible, at even faster exposures than were possible with the SMM! When the resolution is improved dramatically, evidence for localized heating by highly colli- mated particle beams may become available. Also, the higher 10 resolution may permit the extent to which the process of mag- Solar flare seen in X-rays from Skylab (center of sun) netic field annihilation is localized near the flare kernel to be determined. Gaining more information on such key energetic processes will greatly constrain what kinds of physical processes can trigger the flare in the first place. This is essential because the flare trigger could occur on the scale of the ion gyro- radius for a number of plasma instabilities, and this number is unobservably small about 100 cm in the corona.

Ultraviolet image of solar eruption is superimposed on a co- ronogram of this event. Insights into other Stars At least two problems of fundamental importance to stellar sHis, but the currerir stellar picture is wr otizzliri,j For exirne astrophysics are sure to be more amenable to solution when why do some early stars, which are not believed to have con- the Solar Optical Telescope has helped to clarify the situation vective envelopes, have hot coronae, and what initiates the enor- on the Sun. The first is the nature of the dynamos, which gen- mous mass flows observed in the winds of some early 0 and B erate magnetic fields on stars, particularly for stars of spectral stars, Wolf-Rayet stars, and late K supergiants? When the Solar type later than A on the Hertzsprung-Russell diagram, where Optical Telescope has helped to reveal the details of the solar convective envelopes are known to predominate. The second dynamo, of solar atmospheric heating and of the initial accel- is the origin of high-temperature outer atmospheres (chromos- eration of the solar wind in all regions )normal corona, active pheres and coronae) 3nd the origin of stellar winds. In fact, a regions, coronal holes), we can then expect to see many of these body of evidence already suggests that all of these processes insights transferred to an understanding of similar processes on are connected in some complex way, at least for the late-type other stars.

Understanding Late-Star Atmospheric Heating and Stellar Winds Relies Heavily on Knowledge Available from Solar Studies. 11 LATE-STAR OUTER ATMOSPHERES AND WINDS

Class I-b (Supergiants)

-5 ----- AOOl(lO4K)_—. Huge Mass Loss I -- 10- 6 Myr 11) kms Warm ( ' i05 KI -3 )o:13O LLarge Mass Loss ( 10-8 M O yr ' I Moderate( 100 km s L4J ( 0

-1 2 (1

' '.Class Ill (Giants) — Ui D -J +1 -S -.--.- 0

Hot(--106K) +3 Small Mass Loss 10-14 M C yr ' I High Speed I - 500 km s 1)

S.-

SUN -._ Class V(Dwarfs)

+7 AO FO GO KO MO

SPECTRAL CLASS GREGORIAN FOCUS HEAT REJECTING HEAT REJECTING ANNULUS ANNULUS I - SCIENCE I \MIRROR q INSTRUMENT I / /

ENTRANCE PRIMARY APERTURE MIRROR SECONDARY PRIME FOCUS MIRROR

FOLD J/T HEAT DUMP HEAT MIRROR REJECTION MIRROR MIRROR HEAT REJECTION PATH - - GREGORIAN OPTICAL PATH

12 The Telescope Itself - An Efficient Gregorian Configuration

Dramatic advances in science frequently occur simultane- 1.3-meter diameter primary mirror that will be capable of ously with equally dramatic advances in technology. The role achieving diffraction-limited viewing in the visible of 0. I arc of the microscope in elucidating the cellular structure of blood second. Diffraction-limited viewing in the ultraviolet is, in prin- has already been noted. The Solar Optical Telescope program ciple, even better, although technical considerations will will be no exception. The prospect of solving the many im- probably limit this to approximately the same value as in the portant scientific problems described in the preceeding pages visible. Image stability will be achieved by a control system in now seems within our grasp, thanks to the remarkable ad- the telescope, which moves the primary and tertiary mirrors vance in telescope capability that the Solar Optical Telescope in tandem, and will be further enhanced by a correlation will afford when it operates in space. A number of character- tracker in the combined science instrument. istics of the telescope and its mode of operation make this An on-axis Gregorian optical system was chosen for the possible. These characteristics include the large light-gathering Solar Optical Telescope because a field-stop and heat-rejection power of the 1.3-meter diameter primary mirror, the mirror can be located at the prime focus, protecting the subse- remarkable control system that provides high image stability quent optics from concentrated out-of-the-field solar radiation. for comparatively long-duration observations, the high data A small hole at the center of this heat-rejection mirror allows rate at which the science instrument payload can collect high radiation from about a 3-arc-minute diameter area of the Sun resolution spectra and solar images, and the very nature of op- to pass through and impinge on the secondary optics. This erating in space, above the Earth's atmosphere, where the ab- area comprises only about 1 percent of the Sun's 32-arc-min- sence of atmospheric scattering and ultraviolet absorption makes ute diameter disk. The remainder of the solar energy is radi- possible a large number of vitally important measurements that ated away, ultimately out of the front end of the telescope. cannot be made from the ground. Rejection of 99 percent of the incident solar radiation in this The Solar Optical Telescope will be built under the manage- way greatly reduces the tendency for ambient impurities to be ment of the Goddard Space Flight Center (GSFC), with science baked onto the smaller mirrors in the telescope, thus greatly instruments provided by teams led by Principal Investigators. enhancing performance in the ultraviolet. Careful attention to The telescope itself will be built by the Perkin-Elmer Cor- cleanliness, combined with this Gregorian design, will provide poration, and the science instruments selected for the first the greatest possible sensitivity at all wavelengths. flight will be provided by the Lockheed Palo Alto Research Laboratory (LPARL) and the California Institute of Technology, with the actual construction of a combined science instrument taking place at the LPARL. As noted, the telescope will have a The Facility - Science Management, Contamination Control and Accessibility to the Instruments To realize the exciting scientific goals of the Solar Optical scientific community is expected on these later flights. Telescope, the solar physics community and NASA, working The configuration of the Solar Optical Telescope Facility, together, have developed a facility concept for the program. which is illustrated on this page, has evolved, after extensive In practice, this concept will ensure that the telescope and its studies, into a closed cylindrical shell. This configuration ap- science instruments will be developed under NASA pears to be optimum when weight, stiffness, mechanical reso- management for the solar physics community as a whole. The nances, thermal conditioning, minimization of contamination, facility consists of the telescope and these science instruments. and accessibility are all considered together. This shell—the in- The scientific goals for the facility have been set by the Science ner cylinder in the illustration - houses the telescope optics, Working Group (SWG), which comprises the Project Scientist mechanisms, and sensors. It is also designed to support the at GSFC, the Program Scientist from NASA Headquarters, the science instruments, which are mounted externally onto the two Principal Investigators, six Facility Scientists selected from shell. Optimum contamination control is provided by enclos- the worldwide solar community, and one Telescope Scientist, ing the telescope optics within the inner, closed cylindrical who will monitor the development of the telescope. In addi- shell. Optimum accessibility to the science instruments is tion, the Announcement of Opportunity (AO), on which the achieved by mounting them externally to this inner structural selection of the general SWG membership was based, also shell. To further facilitate access to the instruments, the outer specified that some of the observing time should be set aside cylinder is a lightweight nonstructural thermal shroud com- for guest investigators, to be selected from the broadly based posed of 24 individual removeable panels. The overall dimen- solar community at a later date. The SWG and NASA will sions of the facility envelope are 7.3 meters in length and 4.4 jointly plan the Guest Investigator Program for the first flight of meters in diameter (outer cylinder). Thus, the requirements for the facility. NASA is also developing plans for further flights of adding further science instruments to the facility for future the facility, and an even more extensive participation by the flights have already been incorporated into this design. 13

COORDINATED INSTRUMENT PACKAGE (CUP)

WAVE FRONT SENSOR

ORBITER INTERFACE - - SUPPORT STRUCTURE

ARTICULATED PRIMARY MIRROR ASSEMBLY GREGORIAN POD

COMMAND & CONTROL SYSTEM The Scientific Instruments - A Coordinated Instrument Package for Unlocking the Sun's Secrets In order to understand the dynamic solar atmosphere, scien- and velocity in the solar atmosphere. Only when all of these tific instruments are needed that can transform the sunlight data are obtained together over a broad range in atmospheric collected by the telescope into spectra and images of very high height can one perform adequate studies of the fundamental resolution. Only in this way can solar scientists determine physical processes taking place. To achieve this, the imaging values for densities, temperatures, velocities, and magnetic data, such as magnetograms and photographs, can be obtained field strengths in the atmosphere, and only in this way can simultaneously with either visible or ultraviolet spectra, and the they demonstrate what physical processes are responsible for switching between the visible and ultraviolet spectrographs is producing the local magnetic fields, heating the atmosphere, rapid. In this way, it will be possible to study how physical and accelerating the solar wind. Thus, the scientific instru- processes in the lower solar atmosphere (photosphere) are cou- ments that operate with the telescope are the key to produc- pled, through magnetic fields, waves, and bulk motions, to con- ing science data in the form needed to unlock many of the ditions in the outer atmosphere (chromosphere, transition region, Sun's remaining mysteries. corona). Operationally, the CIP can operate in either of two Two science instruments were selected for the first flight of modes: (I) the direct mode in which the entrance beam from the Solar Optical Telescope, using the NASA AO process of the telescope goes directly to the two filtergraphs and does not peer review. These are (I) a coordinated filtergraph/spectro- strike the entrance slit of the spectrograph, and (2) the slit-jaw graph (CES) proposed by the Lockheed Palo Alto Research mode in which the beam strikes the entrance slit and the polished Laboratory )LPARL), and (2) a photometric f:ltergraph instru- slitjaw reflects most of the beam to the filtergraphs, thus allowing ment (PFI) proposed by the California Institute of Technology. spectra and imaging data to be obtained simultaneously. The CFS consists of a visible-light tunable filtergraph/polarime- An important element of the CIP feed optics is the correla- ter, a visible spectrograph, and an ultraviolet spectrograph, tion tracker. Images recorded by the CCD camera of the fil- 14 along with a correlation tracker for maintaining image stabili- tergraph will be read into an image processor that can process ty. The PFI consists of two film cameras that operate with film them in near-real time. By tracking patterns of variable- that is sensitive to visible light and in the near ultraviolet down contrast features, such as the granulation, in the field of view to 2200 A. The PFI will be combined with the CFS to form a observed, the correlation tracker can detect shifts in the image single focal-plane instrument, the coordinated instrument field and provide signals for image stabilization, through ser- package (CIP), which will be built by the LPARL group. The il- voing mirror M4 in the illustration on page 16. This capability lustration on page 16 shows the layout of the CIP. Note that is vital to obtaining magnetograms of high quality near the res- the visible and ultraviolet spectrographs form one major inte- olution limit of the telescope, because magnetograms are grated component. made by subtracting sequential right- and left-polarized images The CIP will be particularly well suited for obtaining ultra- in the image processor. Since the difference between the two high-resolution images and magnetograms for studying the images will be small, it is extremely Important that the solar Sun's magnetic field. It will also obtain high-resolution spectra image remain stable during the recording of the two images. needed for diagnosing local values of temperaturer density, The correlation tracker will do this Parameters of the Coordinated Instrument Package The photographic flitergraph will consist of feed optics. beam-steering optics, and two film cameras, each with its :;n filter wheel The general characteristics of the photomet- ltergraph are: Spectral range: 2200 to 8000 A 0 Spectral bandpass (FWHM): 0.4 to 200 A, depending on filters Field of view: 120 by 160 arc-seconds Spatial resolution (on axis): Diffraction limit of the telescope

St .. a Typical exposure: 0. 1 to 10 seconds I The tunable fiitergraph will consist of a beam distnbuter, t I blocking filters, a tunable red filter, and two CCD cameras of •0 array size to be determined by the availability of CCD chips. p. S p (Either 800 by 800 or 1024 by 1024 arrays will be used.) The tunable filtergraph has the following general characteristics: 0 Spectral range: 4600 to 7600 A Spectrl bandpass (FWHM) 004 A at 4600 ,. 0 25 A at ;1hI 7600 A 15 Field of view (for 1024 by 1024 CCD array) I. Background camera: 1 22 by 122 arc-seconds High-resolution camera: 61 by 61 arc-seconds Spatial resolution (on-axis) ti,T1 Background camera: 0.24 arc-second/pixel ii High-resolution camera: 0.06 arc-second/pixel Typical exposure: 1 second Polarization analyses RCP, LCP. linear I tie The spectrograph system will consist of a slit jaw. a reim- aging mirror, a collimator assembly, visible and ultraviolet grat- ings mounted on a carousel, a Schmidt camera mirror, and two focal planes with four CCD cameras in the visible focal 5. plane and two CCD cameras in the ultraviolet focal plane. The general characteristics of the spectrograph system are: I Spectral range iøe , S Visible: 2800 to J0,000, p Ultraviolet: 1200 to 2000 Spectral baridpass (FWHM) Visible: 0.004 A at 2800 ,. p.01 6 A at 10,000 Ultraviolet: 0.03 A at 1200 A. 0.06 A at 2000 Field of view (slit size): 61 by 0.08 arc-seconds i Spatial resolution (on axis): 0.08 arc-second/pixel Typical exposure: 0.001 to JO seconds Spectra of much higher resolution than I sec are needed Polarization analyses. RCP. LCP, linear for dynamical studies. Coordinated Instrument Package

PHOTOMETRIC FILTERGRAPH

CORRELATION TRACKER I

16 TUNABLE 1 FILTERGRAPH

• AND VISIBLE SPECTROGRAPH

CIP FEED OPTICS

FROM TELESCOPE Science Operations from the Shuttle The Solar Optical Telescope will be the largest science facility is perpendicular to the orbital plane and the Orbiter's three that NASA currently plans to operate from the Shuttle. It will principal axes maintain a constant orientation, less orbital pre- follow in the footsteps of, and benefit from the precedent of, cession, with respect to a nonrotating coordinate system cen- the highly successful solar experiments carried on the manned tered on the Earth. The Tracking and Data Relay Satellite Skylab in 1973. The instruments on Skylab were deployed and (TDRS) Ku-band link will be used for data Transmission to pointed by a large solar-oriented assembly called the Apollo ground. Telescope Mount. The solar experiments were performed by Command and control of the telescope and the science in- astronauts housed in the Skylab who maintained voice con- struments can be accomplished both on board and from the tact with scientists on the ground at the Johnson Space ground. As noted, onboard commands are implemented by Center. In many respects, the operation of the Solar Optical Payload and Mission specialists from the Orbiter Aft Flight Telescope will be similar, with Payload and Mission Specialists Deck, and ground-based commands will originate in a Pay- on board the Orbiter performing much of the "hands-on" im- load Operations Control Center (P0CC). Because the TDRS plementation of experiments, while scientists on the ground link will permit near-real-time two-way communications be- determine which experiments or observing programs should tween the Orbiter and the P0CC, near-real-time data assess- be performed. However, there will be two significant differ- ment and command response from the ground will be possi- ences between these two missions favoring the Solar Optical ble for much of the mission. The experiment command soft- Telescope. The spatial resolution, or magnifying power, of the ware will consist of a reasonable number of high-level com- Solar Optical Telescope will typically be a factor of 30 to 50 mands that will actuate entire observing programs, along with higher than that provided by the Skylab experiments, and the the constituent "building-block" programs from which modi- rate at which scientific data can be collected by the new tele- fied high-level commands can be constructed in rapid re- scope and its detectors will greatly exceed that of the digital sponse to existing observational opportunities. This optimizes recorders on Skylab. the joining of a basic, simple scientific timeline with some flex- Launch and operation of tne Solar Optical Telescope on the ibility to meet changing conditions, a lesson learned during Shuttle is scheduled for the early 1990's. An Instrument Point- the solar observations from Skylab. 17 ing System attached to the Shuttle by means of a Spacelab Video-disk recording of the digital science data is tentatively pallet will point the telescope, and maximum use of Shuttle planned, pending further studies of anticipated technical ad- and Spacelab standard support subsystems is envisioned. The vances in this field, which will include an assessment of gener- first flight is planned as a 7-day mission, during which more al availability of high-quality video-disk readers to the scientific than i012 digital data bits and 50,000 frames of film will be user community. If implemented as planned, guest investiga- accumulated during approximately 100 hours of actual scien- tors will receive the disks from the experimenters and/or NASA tific data collection. To maximize solar observing time, a high- and will perform their scientific investigations at their home in- inclination orbit will be sought. The anticipated Orbiter atti- stitutions, using their locally available video-disk readers and tude is a modification of one of the NASA standard three-axis their own scientific software and computer facilities. stabilized attitudes in which the longitudinal axis of the Orbiter The Dynamic Solar Atmosphere

Only within the past few decades, thanks in part to obser- question that we need to answer in order to understand solar 18 vations made from space, has the dynamic character of the and stellar magnetism. We also need to know the dynamics solar atmosphere been revealed in its presently known as- (movement) of the magnetic field elements both in the large pects. The ubiquity of plasma motions observed thus far has area of bright emission (known as an active region) and out- revealed both a great diversity of phenomena and a stagger- side this area, particularly on the boundary, because this ing complexity of structure, as well as many hints of an under- movement is related to the type of dynamo that generates the lying and unifying picture of the basic physical processes that field. are occurring. A good example of this complexity which sug- In the lower right-hand side of the photograph appear sev- gests unity is the currently known, but poorly understood, pic- eral approximately circular regions of "quiet" atmosphere sur- ture of the Sun's magnetic field. All of the diverse solar features rounded by rings of bright emission. These are the supergran- whose properties are clearly strongly influenced, if not deter- ulation cells, whose dynamics will influence the diffusion of mined, by the magnetic field may ultimately depend on the the magnetic field across the solar surface and whose internal detailed characteristics of the subsurface dynamo which gen- motions are a signature of subsurface convection. Within erates that field. To penetrate to a better understanding of these cells, we see a mottled appearance, with bright specks both the individual features and of the underlying physical of emission showing up here and there. Perhaps these specks processes that produce them, many examples are given here are the rising elements called bright granules that have over- to show that observations on the small scales for energy trans- shot the upper boundary of the turbulent convective region, fer by radiation and gas motions in the solar atmosphere are or perhaps they are rising wave trains generated by lower necessary. lying granulation that is already beginning to dump energy in- The illustration on page 19 reveals some of the features in to the atmosphere as weak shocks. the solar atmosphere that the Solar Optical Telescope will help These and other solar phenomena that occur on scales too us to understand. The sunspot in the lower left-hand corner small to be observed with contemporary solar instruments can has a structured magnetic field of several thousand gauss. be studied adequately only with the capabilities of the Solar Why the magnetic field is stable on this scale, as well as on the Optical Telescope. scale of the smallest bright elements lying outside the spot, is a

Ground-based photograph of large sunspot and associated atmospheric structures. jip

Ira gpu1 pp OF , • 4

4V :' Alp ot 41P a 1 • --

11 w.

'S *1 4Arl :' I (

owl .44^, 4,

Summary . . . THE SOLAR OPTICAL TELESCOPE WILL... . 20 • STUDY THE PHYSICS OF THE SUN ON THE SCALE AT WHICH MANY OF THE PHYSICAL • PROCESSES OCCUR • : ADVANCE OUR FUNDAMENTAL KNOWLEDGE OF MANY SOLAR ASTROPHYSICAL PROCESSES

- magnetic fields • - convective energy transport - atmospheric heating - atmospheric dynamics —flares - S • PROVIDE THE KNOWLEDGE NEEDED TO UNDERSTAND MANY FUNDAMENTAL S • ASTROPHYSICAL PROCESSES ON OTHER STARS

• OPERATE FROM THE SHUTTLE AS NASA's MAJOR SOLAR FACILITY FOR THE EARLY S 1990'S

Looking Ahead •.

• THE SOLAR OPTICAL TELESCOPE BECOMES THE NUCLEUS OF AN ADVANCED SOLAR OBSERVATORY ON THE SPACE STATION.

S Acknowledgments Cover (Front mid Bdck( Perkin-Elmer Corporation Page 3 Chapter 4 by R.G. Athay in 'The Sun as a Star" NASA SP-450 Page 6 A.M. Title and The Sacramento Peak Observatory Page 7 Sacramento Peak Observatory Page 8 G.E.Brueckner and The Naval Research Laboratory Page 9 (top) Harvard College Observatory (middle) American Science and Engineering Company (bottom) Naval Research Laboratory Page 10. (top) American Science and Engineering Company (bottom) Naval Research Laboratory and The High Altitude Observatory Page II A K DuPree in lAU Colloquium No 59 Page 12: The Solar Optical Telescope Project, Goddard Space Flight Center Page 13 The Solar Optical Telescope Project, Goddard Space Flight Center Page 14 Jean-Claude Pecker and Richard N. Thomas Page 15 G E. Brueckner and The Naval Research Laboratory Page 16 AM. Title and The Lockheed Palo Alto Research Laboratory Page 17. Lockheed Palo Alto Research Laboratory Page 19 H. Zirin and The Big Bear Solar Observatory

Text Stuart Jordan, assisted by members of The Solar Optical Telescope Science Working Group and The Solar Optical Telescope Project at Goddard

The Solar Optical Telescope is shown deployed by an Instrument Pointing System which is mounted on a Spacelab Pallet. NASA National Aeronautcs jno Space Administration Goddard Space Flight Center Greenbelt Maryland 2077 1